Patent GB-1396712-A (GB)
Title: Image testing device
IPC: G02B27/46
Filed: 1972-10-26
Abstract: 1396712 Photo-electric focusing systems ERNST LEITZ GmbH 26 Oct 1972 [15 Nov 1971 11 Jan 1972] 49384/72 Heading G1A [Also in Division H4] A focusing system comprises an objective producing an image of an object, a grating situated in the image space of the objective, beam-splitting means to split the image into at least two components, and photo-electric detection means to receive the components of the image modulated by the grating. The output signal has a maximum value when the image is accurately focused on the grating. Movement of the grating relative to the image produces a time varying signal the amplitude of which is maximum for critical focusing. As shown, Fig. 1, an objective lens 2 forms an image of a distant object on a grating 3. A Wollaston prism 4 is arranged to form two polarized image components spaced in the plane of the grating 3 by half a grating constant in a direction perpendicular to the grating lines. A polarizing splitter 5 directs these images, modulated by the grating, to two detectors 6, 7 feeding a differential amplifier 8. The grating acts as a local frequency filter and as the grating is moved relative to the image, e.g. stochastically by movements or vibrations caused by the user of the device, an electrical signal varying at the frequency of the vibration is produced at each detector the amplitude being determined by the intensities of the image components at the local spatial frequency and at other spatial frequencies. The two detectors view image components spaced by half a grating constant and therefore the respective local spatial frequency components of the signals are out of phase whereas the remaining spatial frequency components are in phase since they are not affected by the grating. The output of the differential amplifier 8 is therefore a time varying signal the amplitude of which is proportional to the intensity of the local spatial frequency component in the image, the other spatial frequency components having been cancelled, and when the image is accurately focused on the grating this amplitude has a maximum value. The output of amplifier 8 is applied to an indicator 9 and to a regulator 10 which adjusts the position of the lens 2 to bring the image into sharp focus. Instead of using a polarizing system the spaced images can be produced by colour splitting using a dispersion prism and the modulated images can be separated by a dichromatic splitter. Image splitting can also be achieved by the use of gratings having their lines produced in the form of prismatic structures arranged in alternately orientated rows of sawtooth facets Fig. 3 (not shown) or arranged as triangular shaped ridges Fig. 4 (not shown). In either case image components spaced by half the grating constant are directed to the two detectors. An arrangement using a grating in the form of a pattern of pyramidal peaks which direct image components to four photo-detectors to provide measurements in co-ordinate directions is also described Fig. 5 (not shown). Such a pyramidal structure may be used with only two detectors Fig. 6 (not shown) the arrangement being such that the image components overlap in a central area between the detectors to produce an image which may be viewed by the eye Fig. 7 (not shown). Coarse focusing can be achieved manually while observing the indicator connected to the output of the amplifier and then a switch closed to provide automatic fine focusing. The indicator may comprise a flashing light positioned laterally outside or even within the field of view presented to the eye. A second pair of detectors may be incorporated for use with objects showing a periodic structure parallel to one co-ordinate direction and more unequivocal signals are obtained if the grating constant varies across the width and/or length. The grating can be mounted on resilient supports and vibrated laterally by means of Piezo-electric transducer to produce an alternating output signal Fig. 8 (not shown). The application of the focusing arrangements to SLR cameras is described, appropriate modifications to the pentaprism being made by the inclusion of reflectors to direct the image components produced by the focusing screen (grating) to the photo-cells irrespective of the aperture of the objective lens Figs. 9 to 14 (not shown). Instead of pyramidal gratings two crossed triangular grooved gratings may be used. Reflective gratings may also be utilized. A single detector may be used means being provided to associate it alternately with the grating modulated image component beams.